I installed a wired burglar alarm system in my home back in the early 90's. The window sensors that I used attached were adhesive-backed and have remained in place all these years. I'm trying to remove them now but am having no luck doing so. I've tried using a thin blade scraper with no luck. The top section breaks off but the bottom plastic mounting plate won't budge.

I read where rubbing alcohol is supposed to loosen up adhesive so I can give that a try next but does anyone know of a way that will work? I haven't tried a heat gun as I want to make sure I don't break the glass. Any ideas?

WD-40 might soften the foamy sticky-pad part if they used double sided sticky-pad thingies and you can stand the smell of WD-40 .
I might also consider a sharp chisel if they are all that petrified.

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Used mineral spirits (paint thinner) for years to clean up glue on Formica seams. GooGone is on my shelf and works well. Both are also good for removal of labels. Put on a small paper towel and soak. It will do a good job if repeated.

Neat trick, and pretty cheap: Squirt some hand-sanitizer along the upper edge and tamp it down into the seam with the edge of a spackle/putty knife. Give it about a minute to soften the adhesive and then soak the whole thing in hand-sanitizer and you should find that the putty knife can easily take it off.

Use a razor blade or box knife to cut through the foam, and once the piece has been cut off you'll have clear access to the residual foam to soak in Goo Gone or Goof Off for a couple of hours before easily scraping off at your leisure.

Before using any solution, knowing what it's adhered to is critical. I would apply a small amount of whatever you plan on trying someplace inconspicous before using it, especially if the window frame is plastic or vinyl. Might not damage the window, but could permanently discolor it.